This apparatus is related to a system for measuring pressure along any number of intermediate sites along a conduit such as a medical catheter inserted into a cavity. More particularly, the catheter may be used for simultaneous measurement of the pressure within a urinary bladder and the urethra leading to the bladder.
In a variety of situations involving the urodynamic clarification of the functional processes of the urinary tract and bladder, it is desirable to be able to measure the pressure differential between the urinary bladder and the urethra in order to determine the pressure profile of each particular anatomical element. The Assignee of the present application, Fiberoptic Sensor Technologies, Inc., (FST) has been in the forefront of the development of a variety of fiberoptic-based invasive pressure sensing devices and techniques. The advantages and uses of such devices and techniques are described in a number of U.S. patents previously issued to FST, including U.S. Pat. Nos. 4,711,246; 4,787,396; 4,856,317; and 4,924,870; and pending U.S. patent applications Ser. No. 07/748,082; filed on Aug. 21, 1991; Ser. No. 07/823,143, filed on Jan. 21, 1992; and Ser. No. 07/870,395, filed on Apr. 17, 1992, which are hereby incorporated by reference. The systems described in the above referenced documents generally employ a catheter having a deformable diaphragm positioned near the distal end of an optical fiber. Deflection of the diaphragm in response to external fluid pressure changes its shape and proximity to the end of the optical fiber. A light signal injected into the proximal end of the optical fiber exits the distal end of the fiber and is reflected by the deformable diaphragm to return along the fiber. The shape and spacing of the diaphragm from the fiber end affects the intensity of the returned light. The diaphragm is calibrated to provide a pressure measurement.
Fiber optic pressure sensors of the type described in FST's previously issued patents and pending applications possess a number of fundamental advantages over the previously used approach of fluid pressure measurement which comprises the use of a catheter lumen communicating with a remote site within the body which is connected to an external fluid column type pressure measuring device. These systems possess inherent disadvantages that arise from mechanically coupling a fluid pressure wave through a fluid column imbedded within a catheter to an external transducer. Both the mechanical compliance and the damping losses of the fluid column, the catheter material, and the transducer membrane result in broad resonance artifacts, typically occurring at frequencies in the vicinity of 10 to 20 Hertz and limit high frequency response. Moreover, any extensions of the catheter link used, for example, for a bedridden patient often result in impedance mismatching between tubing and connectors which can create additional resonance peaks. Because significant fluid pressure wave spectral components lie near the resonance frequencies of column sensors, some frequencies will be amplified relative to others, producing a distorted waveform. Waveform distortion is also produced by bubbles trapped in the fluid column. In addition, these types of pressure sensors suffer the disadvantage of distortions that are caused by patient or catheter movement. Motion produces a shift in fluid column position which adds baseline or low fluid frequency artifacts to the pressure waveform. It is for these reasons that direct pressure sensing at the position of the sensor within the catheter is becoming a preferred approach in clinical settings for pressure measurement and is gaining wider acceptance in such applications.
In addition to general fluid pressure monitoring, clinicians who specialize in the evaluation and diagnosis of urinary tract and bladder disorders are often interested in the differential pressure existing between the urinary bladder and the urethra. Such measurements yield a pressure profile which is particularly helpful in determining the closing capacity of specific urethral sections and may be especially important for the clarification of urinary incontinence. One commonly used method of measuring the pressure differential between the urinary bladder and the urethra generally includes the disposition of pressure sensor measurement means along the length of a catheter inserted into the urethra and the urinary bladder. The intermediate pressure sensing means generally comprises a balloon catheter which transmits pressure exerted within the urethra or the urinary bladder through the balloon to exert force on a pressure sensing measurement device. Such balloon catheters are not generally recommended for urethral pressure measurement because they cause spasms, yielding an inaccurate determination of urethral pressure measurements. Moreover, such balloon catheters generally operate on fluid column devices which include the inherent disadvantages as listed above as compared to fiberoptic measurement in determining fluid pressure.